Fix host upload: handle multer errors as JSON, pre-create upload-tmp dir; v2.0.41

If multer failed to write the temp file (e.g. data-volume permission issue),
the error was propagating unhandled through Express, returning a non-JSON 500
that the client couldn't parse — showing only "Upload failed." with no details.

Changes:
- Import mkdir and pre-create UPLOAD_TEMP_DIR on startup (non-blocking)
- Wrap upload.single('file') in a callback so multer errors return JSON
  responses with the actual message instead of a bare HTML 500

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "siteforge",
   "private": true,
-  "version": "2.0.40",
+  "version": "2.0.41",
   "type": "module",
   "scripts": {
     "dev": "vite",